This invention relates to a driving circuit of a display device and a liquid crystal display device. More particularly, the invention relates to those having a simple structure, ensuring representation of good-quality images, and very easy to change the gradation of display.
As one type of flat displays used in personal computers, thin-type television receivers, etc., there is a display using a plurality of pixels having a capacitive load. An example thereof is liquid crystal displays.
So-called xe2x80x9cactive matrix type liquid crystal displaysxe2x80x9d using a pixel switching element like thin-film transistor (TFT) or thin film diode (TFD) for each pixel provide clear images, and have a high-density display ability equivalent to or higher than that of CRT. In particular, TFT liquid crystal displays (TFT-LCD) using thin film transistors as pixel switching elements are under active developments toward their practical use.
TFT typically uses amorphous silicon or polycrystalline silicon as its semiconductor active layer (channel, source and drain regions). For years, vigorous energy is being paid toward development of TFT-LCD of a driving circuit built-in type, in which scanning line driving circuits and video signal line driving circuits are formed together with pixel TFT on a transparent insulating substrate. This structure enables extending the effective display area of the transparent insulating substrate of a liquid crystal display and reducing the manufacturing cost.
TFT-LCD of this driving circuit built-in type includes on its pixel substrate a digital-analog converter circuit (hereinafter abbreviated DAC) for converting digital signals input as video signals from outside into analog signals.
However, conventional driving circuit built-in TFT-LCDs involved the problem that when the bit number of digital input signals was increased for the purpose of attaining higher definition of display images, DAC was enlarged in size and hence narrowed the effective display area of the screen. This problem is discussed below with reference to the drawings.
FIG. 56 is a conceptional diagram showing the structure of a capacitor-arrayed DAC used in conventional liquid displays. DAC shown here is of a parallel input type and includes a switch control circuit 41, reference voltage source 42, switch array 43, capacitor array 44, reset switch 45 and buffer amplifier 46.
In the example shown here, digital data of 6 bits such as (B6, B5, . . . B1) are input in parallel as video signals.
The capacitor array 44 includes capacitors more by one than the number of bits of the digital data. Capacitance values of these capacitors are weighted in six different values from C through C/32 depending upon the binary coding. These capacitors are commonly connected at one end of each thereof, and connected to a vides signal line via the amplifier 46. The capacitors are connected at the other end of each thereof selectively to a reference voltage Vs or the ground potential by MOS switches of the switch array 43.
Each switch of the switch array 43 is directly controlled by binary data of an input which coincides with the capacitance weighting order of the capacitors.
In the example shown in FIG. 56, 6-bit conversion is possible. That is, when the parallel data (B6, B5, . . . B1) are input, the output voltage Vout is expressed by the following equation.                               V          out                =                              ∑                          i              =              1                        6                    ⁢                      xe2x80x83                    ⁢                                    B              i                        ·                          2                              (                                  i                  -                  7                                )                                      ·            Vs                                              (        1        )            
This DAC, however, needs (n+1) capacitors for converting digital data of n bits. Therefore, in order to ensure high definition image display with a high display gradation by increasing the number of bits, the problem of a dimensional increase of the circuit inevitably occurs. In case of liquid crystal displays having a built-in driving circuit, dimensional increase of the DAC circuit makes it difficult to keep a sufficient effective pixel area, and causes the problem that the size of the display need be increased, and the weight becomes heavy.
On the other hand, in DAC of FIG. 56, since the capacitors in the capacitor array 44 must be weighted in capacitance so as to correspond to the binary coding, more and more accurate control of their capacitance values must be guaranteed as the number of bits increases. Therefore, the design and manufacturing margins are strict, and the production yield is liable to decrease.
Another problem with conventional DAC as shown in FIG. 56 lies in that digital data it can convert is fixed in number of bit. That is, the gradation of video signals it can handle is fixed to a predetermined value according to the circuit arrangement of DAC, and it cannot be changed later. This means, in a personal computer, for example, that users cannot readily change the display mode depending on the content to be displayed.
Under the circumstances, Japanese Patent Laid-Open Publication No. H7-72822 includes description about the use of serial DAC made up of two capacitor elements. This structure, however, was limited in increasing the data processing speed because it required different periods for digital-analog conversion and input of analog signals into capacitor elements or output of digital signal from the capacitor elements.
The invention has been made starting with the recognition of the above-explained problems. It is therefore an object of the invention to provide a display device driving circuit and a liquid crystal display device enabling the use of a small-scaled circuit, ensuring good-quality images and enabling free changes of the display gradations.
To attain the object, the basic structure of the invention is made up of a reference voltage selecting circuit for exclusively selecting and outputting one of a plurality of reference voltages in response to individual bit signals of serial data of a plurality of bits which are time-serially input, a first capacitor element connected to the reference voltage selecting circuit to hold a reference voltage output from the reference voltage selecting circuit, a second capacitor element connected to the first capacitor element through a connection circuit to hold an electric charged distributed from the first capacitor element due to a short-circuit of the connection circuit at a timing prior to the input of individual signals into the reference voltage selecting circuit, and an output line which outputs the voltage held in the second capacitor element as a display signal.
That is, the display device driving circuit according to the invention is one configured to input digital data and output it after convert it into an analog video signal, which comprises a first capacitor, a selecting circuit introducing one of bits of the digital data to determine the charge voltage of the first capacitor as a first voltage when the value of the bit is xe2x80x9c1xe2x80x9d but determine the charge voltage of the first capacitor as a second voltage different from the first voltage when the value of the bit is xe2x80x9c0xe2x80x9d, a second capacitor, and a connection circuit which connects the first capacitor and the second capacitor to reallocate their charging electric charges so as to equalize their charging voltage, such that the charging voltage of the first capacitor or the second capacitor obtained by activating the selecting circuit and the connection circuit in this sequence for each of the bits of the digital data from its most significant bit to the least significant bit be output as the analog video signal.
Based on the basic structure shown above, the first display device driving circuit according to the invention is a driving circuit of a display device including a digital-analog converter circuit which introduces a digital data and outputs an analog video signal, in which the digital-analog converter circuit has an input capacitor parallel type structure including: a reference voltage selecting circuit which is responsive to each of bit signals of time-serially input data of a plurality of bits to exclusively select and output one of a plurality of reference voltages; a group of input-side capacitor elements connected to the reference voltage selecting circuit and including a plurality of capacitor elements which hold the reference voltage output from the reference voltage selecting circuit; an output-side capacitor element connected to each capacitor element in the group of input-side capacitor elements via a connection circuit to short-circuit the connection circuit at a predetermined timing, thereby selectively connected to individual capacitor elements in the group of input-side capacitors sequentially and holding charges distributed from individual capacitor elements in the group of input-side capacitor elements, thereby outputting the voltage held in the output-side capacitor element as an analog video signal.
The second display device driving circuit according to the invention is a driving circuit of a display device including a digital-analog converter circuit which introduces digital data and outputs an analog video signal, in which the digital-analog converter circuit has an output capacitor parallel type structure including: a reference voltage selecting circuit which is responsive to each of bit signals of time-serially input data of a plurality of bits to exclusively select and output one of a plurality of reference voltages; an input-side capacitor element connected to the reference voltage selecting circuit to hold the reference voltage output from the reference voltage selecting circuit; a group of output-side capacitor elements connected to the input-side capacitor element via a connection circuit to short-circuit the connection circuit at a predetermined timing, thereby connected to the input-side capacitor element and holding charges distributed from the input-side capacitor element, thereby selectively outputting a voltage held in a capacitor element in the group of output-side capacitor elements as an analog video signal.
The third display device driving circuit according to the invention is a driving circuit of a display device including a digital-analog converter circuit which introduces digital data and outputs an analog video signal, in which the digital-analog converter circuit has an input-output capacitor parallel type structure including: a reference voltage selecting circuit which is responsive to each of bit signals of time-serially input data of a plurality of bits to exclusively select and output one of a plurality of reference voltages; a group of input-side capacitor elements connected to the reference voltage selecting circuit and including a plurality of capacitor elements which hold the reference voltage output from the reference voltage selecting circuit; a group of output-side capacitor elements connected to respective capacitor elements in the group of input-side capacitor elements via a connection circuit to short-circuit the connection circuit at a predetermined timing, thereby selectively sequentially connected to individual capacitor elements in the group of input-side capacitor elements and holding charges distributed from individual capacitor elements in the group of input-side capacitor elements, thereby selectively outputting a voltage held in-a capacitor element in the group of output-side capacitor elements as an analog video signal.
The first to third driving circuits may further comprise a shutoff circuit connected between the reference voltage selecting circuit and the input-side capacitor to cut the connection between the reference voltage selecting circuit and the input-side capacitor element before short-circuiting the input-side capacitor element and the output-side capacitor element. Thereby, a back flow of the electric charge from the input-side capacitor element to the reference voltage selecting circuit can be prevented.
The input-side capacitor element and the output-side capacitor element may be substantially same in capacitance value to ensure even reallocation of the electric charge.
On the other hand, the fourth display device driving circuit according to the invention is a display device driving circuit having a plurality of signal lines and scanning lines arranged to intersect at right angles with each other and pixel switching elements provided at crossing points of the signal lines and the scanning lines to display gradient representation of 2m on the basis of data of m bits (m is a plural number), comprising: a data distributing circuit supplied with the data of m bits; a data latch circuit sequentially storing the data of n bits and outputting them at a predetermined timing; a gamma correction circuit which stores the output from the data latch circuit and outputs it at a predetermined timing; one of the digital-analog converter circuits recited in claims 1 through 9 which stores the output from the gamma correction circuit and outputs it at a predetermined timing; and an amplifier circuit for amplifying the output from the digital-analog converter circuit.
On the other hand, a liquid crystal display device according to the invention comprises one of the above-summarized display device driving circuits, and a liquid crystal controlled by the pixel switching elements, and it is characterized in that the threshold value of the liquid crystal is about 2.5 Volts.
Another liquid crystal display device according to the invention comprises one of the above-summarized display device driving circuits, and a liquid crystal controlled by the pixel switching elements, and it is characterized in that the threshold value of the liquid crystal is about 1.5 Volts.
Another liquid crystal display device according to the invention is a transmission type liquid crystal display device comprising one of the above-summarized display device driving circuits, and a light source provided behind when viewed from the direction of the image watching surface.
Another liquid crystal display device according to the invention is a reflection type liquid crystal display device comprising one of the above-summarized display device driving circuits, and a reflector provided behind when viewed from the direction of the image watching surface to display images by reflecting external light entering from the direction of the image watching surface with the reflector.
Another liquid crystal display device according to the invention comprises one of the above-summarized display device driving circuits, a light source provided behind when viewed from the direction of the image watching surface and a reflector provided behind when viewed from the direction of the image watching surface, and it is characterized in representing images by letting light released from the light source to pass through or by reflecting external light entering from the direction of the image watching surface with the reflector.
Another liquid crystal display device according to the invention comprises one of the above-summarized display device driving circuits, and a pixel switching element provided for each display pixel, and it is characterized in that the driving circuit and the pixel switching elements are provided on a common substrate and share a common semiconductor layer stacked on the substrate.
The invention is used in the above-summarized modes, and gives the effects explained below.
First of all, according to the invention, serially input digital video signals are reliably and easily converted into analog signals. Additionally, according to the invention, the circuit arrangement of DAC is very simple, and the circuit area can be reduced more largely than conventional ones. The dimensional reduction of the circuit becomes more effective with the number of bits of digital data. Namely, as the quality of the displayed images becomes higher, this effect becomes larger.
That is, as compared with typical n-bit parallel input type DAC, the circuit scale of DAC in the present invention is about 1/n, and as the number of bits increases, the effect of dimensional reduction of the circuit becomes larger. This is especially advantageous in polysilicon TFT liquid crystal display devicees integrating the driving circuit into the panel. Although an increase in display gradation, i.e. number of bits of video signals, is necessary for higher quality of displayed images, the invention need not increase the circuit scale, and simultaneously reconciles miniaturization of the panel and improvement of the image quality.
Further, the invention also enables conversion of digital data different in number of bits into analog video signals without changing the circuit. That is, according to the invention, by repeating the above-explained operations for individual bits of serially input digital data, it is possible to convert the digital data into an analog form without relying on the number of bits.
This effect of the invention is especially advantageous when it is applied to display devicees of computers. That is, in computers, it is often required to switch the image display mode depending upon its use or software. In that case, it is desirable to change the display gradation, i.e. number of gradation bits, together with its display resolution. According to the invention, even in that case, analog conversion is possible by using the same DAC.
Furthermore, according to the invention, digital data can be input in parallel at a high speed by providing a plurality of primary-side capacitors of DAC. In addition, by providing a plurality of secondary-side capacitors of DAC, it is possible to execute in parallel the DA conversion for the next signal line and writing of the analog potential to the preceding signal line. This results in increasing the operation speed and enabling reliable writing of a predetermined analog potential even when the parasitic capacitance of the signal line is large in a high-definition display device.
Moreover, according to the invention, as the output circuit controls to increase the voltage of a signal line when the voltage of the signal line is lower than the voltage of the input signal and decrease the voltage of the signal line when the voltage of the signal line is higher than the voltage of the input signal, the voltage of the signal line can be equalized to the voltage of the input signal.
Furthermore, by setting the voltage of the input terminal of each inverter forming the output circuit at the threshold voltage of its own before the output circuit controls the voltage of the signal line, adverse affection to the voltage of the signal line can be prevented even when the inverters fluctuate in threshold voltage.
As explained above, according to the invention, it is possible to reliably write digital data different in number of bits onto signal lines after converting them into analog video signals by using a much simpler circuit arrangement than conventional ones, and its industrial advantages are great.